Process of protecting surfaces of aluminum or aluminum alloys



Patented July 29, 1930 UNITED STATES PATENT OFFICE GUY DUNSTAN BENGOUGH AND JOHN MOARTI-IUR STUART, OF LONDON, ENGLAND PROCESS OF PROTECTING SURFACES OF ALUMINUM OR ALUMINUM ALLOYS No Drawing. Application filed July 28, 1924, Serial No. 728,765, and in Great Britain August 2, 1923'.

This invention relates toan improved process of producing a resistant coating, by oxi dation of the metal when made an anode, on the surface of aluminum or aluminum alloys 5 by anodic treatment in an electrolytic bath.

ACCOICliIlg to the invention, such a coating is produced by anodic treatment in a bath consisting of an aqueous solution ol chromic acid.

The conditions of concentration and temperature may be varied accordingly as aluminum itself or any particular alloy of alumium is under treatment.

In the case oi the metal known under the registered trade mark duralumin or ot commercial sheet aluminum the following directions give the best result at present known to us.

The object is first Washed in a solvent for grease and then in hot Water; it is then made the anode in a bath consisting of an aqueous solution of chromic acid of 3 per cent strength (the chromic acid being calculated as C-rOQ), which should be free from sulphuric acid and sulphates. The cathode may be of carbon.

The immersed surface should consist entirely of aluminum or aluminum alloy, the electrical connections being made so that no other metal dips into the bath. If necessary the object may be partially immersed and treated and then inverted so that the remainder of the surface is now immersed and becomes treated.

The temperature of the bath is kept at 40 C. at least and the bulk may be stirred dun ing the treatment. The voltage must be carefully regulated to suit the other conditions; when the latter are those already named the following procedure has been found suitable :Raise the voltage across the bath gradually to 40 volts in 15 n1inutes,l;eep at l volts for 35 minutes, and then raise the voltage to 50 volts in the course of 5 minutes and retain it at this value tor 5 minutes. The object is then Washed and dried.

The film or coating produced by this insuch liquids EIStSCtt-WfltOl resist pitting and o her forms of attack for long periods.

The degree of resistance I is greatly iii-- creased if the coated surface is greased or oiled; for the coating has great power ol adsorption and the grease becomes incorporated in the surface. For this reason the coated surface is well adapted for being; painted since the oil of the paint becomes ad sorbed. Lanoline is a particularly suitable grease for treating the coated surface; it may he applied in liquid form, in solution or as an emulsion.

As will be clear from the foregoing description. the invention is applicable both to surl aces purely of aluminum and to surfaces comprising aluminum in the. form of an alu minum alloy, and it is accordingly to be understood that in the ap 'iended claims, the expression predominantly of aluminun'i is in tended to include both aluminui'n itself and alloys of aluminum.

Having thus described the nature of the said invention and the best means We know of carrying the same into practical effect, We claim A process of producing a resistant coating by oxidation on a surface predominantly of aluminum, which process consists in washing the surf ace with a solvent for grease and then in hot water, making the Washed surface the anode in an electrolytic bath consisting of an aqueous solution oi? chromic acid of 3 per cent strength and containing a carbon cathode, maintaining the tei'nperature of the bath at a temperature not less than =l0 (1 raising the voltage'across the bath to about l0 volts in the course of about 15 minutes. keeping the voltage at this value for about minl zl IOU 

